A case-control study was performed in order to examine the relation of cigarette smoking and alcohol use to colorectal carcinoma in situ and cancer, separately. Study subjects consisted of 129 colorectal carcinoma in situ cases, 66 colorectal cancer cases, and 390 controls recruited from health check-up examinees in Tokyo from January 1991 to March 1993. Smoking status and alcohol habit were ascertained from a self-administered questionnaire. Both cumulative cigarette smoking and current smoking status were associated significantly with an increased risk of carcinoma in situ. A statistically nonsignificant increase in the risk of colorectal cancer was noted only among those with the heaviest exposure category of these measures of smoking. The cumulative exposure to cigarette smoking within recent 20 years was associated significantly with an increased risk of carcinoma in situ, whereas smoking until 20 years before the diagnosis was associated significantly with an increase of cancer risk. A significant and positive association was observed between cumulative alcohol drinking and colorectal cancer. These findings suggest that cigarette smoking may act as an initiator in colorectal carcinogenesis and also provide weak evidence that alcohol drinking is related to an increased risk of colorectal cancer.